1. Field of the Invention
This invention is concerned with an improvement in or relating to apparatus for mass spectrometry and especially for such apparatus that can achieve high accuracy, better than +/-0.2%, at ultra-high sensitivity, better than one part per trillion (10.sup.-12), for the measurement of the ratios of rare isotopes, such as .sup.14 C, to the abundant isotopes, in this case .sup.12 C and .sup.13 C.
2. Review of the Prior Art
There is a continuing need to increase the accuracy of the measurements of rare naturally occurring isotopes such as .sup.14 C in small (1 milligram) samples of carbon. These measurements require the addition of a tandem accelerator, after the low energy negative ion mass spectrometry, to accelerate the ions to a few million electron volts so that all molecular interferences, such as .sup.12 CH.sup.-.sub.2 and .sup.13 CH.sup.-, can be eliminated. This elimination is accomplished by the passage of the ions through a long tube (typically 6.6 mm diameter and 800 mm long) of higher gas pressure, known as a stripping canal, located in the center of the tandem accelerator where the ions loose four electrons to become triply charged positive ions These ions are then accelerated further by the accelerator and analyzed by another mass spectrometer system prior to counting the individual ions. This procedure for the destruction of molecular interferences is the basis of U.S. Pat. No. 4,037,100 for mass spectrometry issued on July 19, 1977. The general features of a recombinator for ions differing in energy and time of flight (known as an Isochronator) are described in U.S. Pat. No. 4,489,237 dated Dec. 18, 1984. The device described herein is a practical version of a mass recombinator with mass dispersion and selection midway through the system and spatial recombination of all masses at the end. What the Isochronator referred to in the above mentioned patent does for ions with differing flight time through the spectrometer, this device does for ions of differing mass.
In another development some isobaric interferences were also eliminated by exploiting the negative ion instability of one member of the isobaric pair. For example, the .sup.14 C negative ion is stable whereas the .sup.14 N negative ion is so unstable that it is not transmitted through the mass spectrometer.
Several devices based upon the principles outlined above have been constructed successfully. However the use of a stripping canal and the analysis of one isotope of carbon at a time reduces the time for analysis of the rare carbon isotope, .sup.14 C, and introduces the possibility of errors due to the variable ion transmission probability through the system.
Conventional mass spectrometers have for many years exploited the simultaneous measurement of several isotopes to improve the accuracy and shorten the analysis time. A system to inject isotopes simultaneously into a tandem accelerator has been described. This system disperses and recombines the different mass isotope beams in direction only and each ion beam is focused to a different point in space. As all ions must pass through the same narrow stripping canal, this feature is undesirable for high accuracy work and a system which first disperses all isotopes for mass analysis and then recombines them precisely at the same point prior to injecting them into the accelerator is necessary for the highest accuracy work.
There is a need for rare isotope analysis system which can: (a) determine isotope abundances between the parts per trillion (10.sup.-12)and the parts per quadrillion (10.sup.-15) region, (b) have high sensitivity and low background to reduce counting time for each sample or to achieve the maximum measurement accuracy in the shortest time.